1. Field of the Invention
This invention relates generally to the field of holography and more particularly to a novel multifrequency laser oscillator which generates coherent light of two closely adjacent optical frequencies by rapid succession for recording contour holograms by the double exposure technique.
2. Prior Art
As is well known to those versed in the art of holography, holographic contouring involves recording a hologram of an object in such a way that the holographic image reconstructed from the hologram displays dark contour lines or bands, commonly referred to as contour fringes, from which the surface contour of the object may be determined with relatively high accuracy. The basic underlying principles of holographic contouring and the mechanics of implementing these principles are known and described in the open literature. For example, the basic principles of holographic contouring and holographic contouring apparatus are described in U.S. Pat. Nos. 3,603,684 and 3,683,685. Accordingly, it is unnecessary to elaborate on the basic principles of contouring in this disclosure.
Suffice it to say that holographic contouring involves holographically recording an object at two different holographic recording medium. The recordings produced by the two frequencies create an interference pattern in the reconstructed holographic image which produces the contour fringes. As discussed in the above mentioned patents, the contour fringe spacing is a function of the difference between the wavelengths of the two coherent light frequencies.
These patents also discuss the fact that a contour hologram may be recorded on a recording medium by a single exposure of the medium at both wavelengths of coherent light simultaneously or by double exposure of the medium, i.e. exposure of the recording medium first at one coherent light frequency and then at the other light frequency. In the single exposure technique, the two light frequencies are generated simultaneously and recorded simultaneously on the recording medium. In the double exposure technique, the two light frequencies are generated and recorded separately on the recording medium. The two methods produce the same contour fringe pattern. This invention is concerned with the double exposure technique.
When utilizing the double exposure technique to record a contour hologram, it is imperative that the object being recorded remain absolutely stationary between the successive exposures; that is to say, the object must remain stationary within a fraction of a wavelength of the coherent light. Accordingly, the double exposure technique is limited in application. Moreover, even in those applications for which the double exposure technique is suitable, the time interval between the successive exposures should be as brief as possible to minimize the possibility of movement of the object between exposures.
While this requirement that the object being recorded remain absolutely stationary between exposures restricts the useful applications of the double exposure contouring technique, the latter has a compensating advantage over the single exposure technique. This advantage stems from the fact that maximum contouring accuracy, and in some cases even the creation of contour fringes, may require different reference beam angles for the two light frequencies.
The single exposure technique presents the difficult task of separating the two frequency components of the single output beam from the coherent light source to provide separate reference beams of these two frequencies, respectively, which may then be directed to the recording medium at the correct interference beam angles. The double exposure technique avoids this problem since the two coherent light frequencies are automatically separated in time, and the holographic optics may be adjusted between exposures to obtain the correct reference beam angle for each frequency.
It is significant to note in connection with this reference beam angle requirement, that if the difference between the two coherent light frequencies is sufficiently small, satisfactory contour holograms may be recorded with the same reference beam angle for both light frequencies. The present invention is described in connection with contour hologram recording utilizing the R.sub.1 and R.sub.2 transitions or wavelengths of the resonant fluorescence of a ruby lasing medium. The frequency difference between these transitions is not sufficiently small to permit the use of the same reference beam angle for both frequencies.
From the discussion to this point, it is apparent that the single and double exposure techniques of recording contour holograms require different types of coherent light sources. Thus, the single exposure technique requires a coherent light source capable of generating two selected optical light frequencies simultaneously. The double exposure technique, on the other hand, requires a coherent light source capable of generating the two light frequencies individually, in rapid succession. In earlier mentioned U.S. Pat. Nos. 3,603,684 and 3,603,685, as well as U.S. Pat. No. 3,492,600, describe coherent light sources of these two different types.